1. Field of the Invention
The present invention is concerned with a film forming process, and more particularly it relates to a process for forming films such as for example a photoconductive film, semiconductor film, inorganic insulating film and organic resin film, by means of a glow discharge.
2. Description of the Prior Art
When a film having desired properties is formed on a substrate in such a manner that source gas for forming a film is decomposed by utilizing the plasma phenomenon, and particularly in case of forming such a film of a large surface area, it is very difficult to obtain a film provided with a uniform thickness as well as equal physical properties such as electric, optical and photoelectric properties at the every portion extending over the entire surface area of the film, as compared with the conventional vacuum deposition. For example, formation of a film of amorphous hydrogenated silicon (hereinafter represented by "a-Si:H") is considered. When an a-Si:H film is formed on a substrate by utilizing discharge energy to decompose SiH.sub.4 gas and the electric physical property of the film is utilized, it is necessary to achieve equal or uniform intensity of the discharge in the entire area for the film formation for the purpose of obtaining equality or uniformity of electric physical property at the every portion covering the entire area of the film since the electric physical property depends greatly upon the intensity of the discharge emloyed during the film formation. However, it is very difficult to achieve uniform intensity of the discharge in the conventional process, particularly when a film of a larger surface area is formed.
Further, in general, the glow discharge deposition utilizing the plasma phenomenon is slow in the speed for forming a film as compared with usual vacuum vapor deposition. Due to this, in connection with the glow dischrge deposition, it is proposed to increase the flow amount of gas to be introduced into a deposition chamber. However, in the conventional process, it is difficult to attain equal flow amount of gas in the whole area for the film formation. Therefore, when the flow amount of gas is increased to attain high speed for forming a film, the flow amount of gas becomes unequal depending upon the spot and ultimately it is considerably difficult to obtain uniform thickness as well as equal properties in the entire area of a film.
Moreover, the uniformity or equality of the thickness and properties of a film to be formed depends greatly upon the positional relation between electrodes and a substrate for forming a film as well as the structure of the electrode. The conventional process is not necessarily satisfactory in this respect and therefore is unsuitable for repeating the film formation of a larger surface area and mass-producing a film having equal properties at every portion.
In view of the foregoing, the conventional process involves drawbacks as mentioned above, and particularly in the conventional process problems concerning the reproducibility and mass-producibility of a film is indicated informing a film having a larger surface area provided with uniform properties and thickness.